Charger and cable power tester

Because the phone is spending power at the same time as it is receiving it, and what the battery percentage shows is only the difference. With the screen on, a phone can draw two to four watts for itself; a tool that reports the gain as the charger’s output therefore understates every charger by that much. The unplugged run measures that appetite directly, under exactly the same conditions, and adding it back gives what is actually arriving through the cable. It is the whole reason this tool asks for two runs rather than one.

Three reasons, all real. The figure on a charger is its maximum, delivered only when the phone asks for it — and a phone asks for full power only in a certain band of charge and temperature. Some energy is lost as heat in the cable, the connector and the phone’s own charging circuit, which never reaches the battery at all. And above roughly 80% the battery itself tapers the current no matter what is offered. A 20 W brick putting 14 or 15 W into the battery in the middle of the charge curve is behaving normally.

Because a percentage is a fraction, not an amount. Gaining one per cent a minute means 12 W in a 20 Wh battery and 7 W in a 12 Wh one, and the browser is never told which it has. The capacity in milliamp-hours is in every phone’s specifications; it is converted here at 3.85 V, the nominal voltage of a modern lithium-polymer cell. Without the capacity everything still works in percent per hour, which is enough to compare two chargers on the same phone.

Because Safari removed the Battery Status API, and so did Firefox — for fingerprinting reasons, since the exact charge level is a surprisingly good way to recognize a returning visitor. Chrome and Edge still have it on Android and on Windows. On iOS every browser is Safari underneath, so there is no way round it from a web page, and the tool says so instead of showing an empty dial.

Nothing went wrong with the runs — they were just taken at different points on the charge curve. A lithium battery accepts full current up to around 80% and then reduces it progressively, so a run measured at 85% will read low however good the charger is. Comparing it with one measured at 40% tells you about the battery, not about the chargers. Take both runs between 20% and 70% and the comparison becomes fair.

Keep everything else fixed and change only the cable: same charger, same socket, same screen brightness, same part of the charge curve. Thin conductors and long runs drop voltage, and the phone responds by drawing less current, so a poor cable shows up as a plainly lower rate. This is the most common hidden cause of “it used to charge faster than this”.

Because the browser reports whole percent, so the tool can only learn something when the number moves. At 120% per hour that is every thirty seconds and five steps take under three minutes; at 15% per hour a step takes four minutes and the same five steps take twenty. The test waits for the steps rather than for a clock, because a rate computed from one step would be a guess with an uncertainty as large as itself.